1. Field of the Invention
The present invention relates to a light pulse generator which can generate light pulses at high power and is used in optical apparatuses such as in an OTDR (Optical Time Domain Reflectometer).
2. Background Art
FIG. 5 is a block diagram showing the configuration of an example of a conventional light pulse generator.
In FIG. 5, an erbium-doped optical fiber 1 is an optical fiber having an erbium-doped core.
A pumped-light source 2 is a light source for continuously emitting a pumped-light at a constant power.
A light mixer 3 has input terminals 3A and 3B and an output terminal 3C. The light mixer 3 mixes input signal lights of the input terminals 3A and 3B, and outputs the mixed light from the output terminal 3C. The pumped-light emitted from the pumped-light source 2 is supplied to the input terminal 3A of the light mixer 3.
A light isolator 7 is inserted between the output terminal 3C of the light mixer 3 and a terminal 1A of the erbium-doped optical fiber 1. The role of the light isolator 7 is to control the flow of the signal light. That is to say, the right direction in FIG. 5 is a forward direction of the light isolator, and the left direction in FIG. 5 is a reverse direction of the light isolator. Therefore, the light isolator 7 transmits a signal light from the light mixer 3 to the erbium-doped optical fiber 1 with no loss or at a very low loss. In contrast, a signal light which is outputted from the erbium-doped optical fiber 1 is attenuated by the light isolator 7. Thus, the flow of the signal light from the erbium-doped optical fiber 1 to the light mixer 3 is prevented.
The other terminal 1B of the optical fiber 1 is connected to an input terminal of an optical switch 5. The transmission loss of signal light of the optical switch 5 is controlled based on an electric control signal S.sub.c. When the level of the control signal S.sub.c is high, the optical switch 5 is in an ON-state. When the level of the control signal S.sub.c is low, the optical switch 5 is in an OFF-state. An optical switch controller 6 supplies the control signal S.sub.c to the optical switch 5 to control the ON/OFF state of the optical switch.
A light divider 4 has an input terminal 4C and output terminals 4A and 4B. The input terminal 4C is connected to the output terminal of the optical switch 5 via an optical fiber. The output terminal 4A is connected to the input terminal 3B of the above-described light mixer 3. The output terminal 4B is a light pulse output terminal of the light pulse generator from which light pulses P.sub.op are sequentially outputted.
Next, description will be given with respect to the operation of the light pulse generator shown in FIG. 5.
FIG. 6A shows an example of a waveform of the control signal S.sub.c outputted from the optical switch controller 6. Periodic pulses having a rectangular waveform are outputted as the control signal S.sub.c as shown in FIG. 6A.
When the level of the control signal S.sub.c Is low, the optical switch 5 Is in the OFF-state. Therefore, the following operation is carried out.
The pumped-light emitted from the pumped-light source 2 is supplied to the input terminal 3A of the light mixer 3. This pumped-light is then outputted from the output terminal 3C of the light mixer 3 and the pumped-light thus outputted is supplied to the erbium-doped optical fiber 1. Energy is accumulated in the erbium-doped optical fiber 1 due to the pumped-light thus supplied. However, the optical switch 5 is in the OFF-state. Therefore, no signal light is supplied to the light divider 4 from the erbium-doped optical fiber 1.
When the level of the control signal S.sub.c is changed to high, the optical switch 5 turns to the ON-state. As a result, an optical loop including the light mixer 3, the optical isolator 7, the erbium-doped optical fiber 1, the optical switch 5, and light divider 4, is closed. The level of the control signal S.sub.c remains high for a short period of time, as shown in FIG. 6A. While the control signal S.sub.c remains at a high level, a signal light having a wavelength band of 1.55 .mu.m is outputted from the terminal 1B of the erbium-doped optical fiber I and the signal light passes through the optical switch 5. As a result, a light pulse is obtained from the output terminal of the optical switch 5. This light pulse is supplied to the light divider 4 and the light pulse thus supplied is then divided by the light divider 4.
The output light pulse obtained from the output terminal 4B of the light divider 4 is supplied to an external device (not shown) as an output light pulse P.sub.op.
The output light pulse obtained from the output terminal 4A is supplied to the input terminal 3B of the light mixer 3. The light pulse thus supplied is supplied to the erbium-doped optical fiber 1 via the light mixer 3 and the light isolator 7.
This causes an increase in the level of amplitude of the signal light outputted from the terminal 1B of the erbium-doped optical fiber 1. That is to say, a positive feedback amplification is carried out in the optical loop. Thus, the level of amplitude of the light pulse obtained from the optical switch 5 is increased.
The light pulse outputted from the optical switch 5 is divided by the light divider 4 and one of the divided light pulses is outputted from the output terminal 4B. As a result, the level of amplitude of the light pulse P.sub.op obtained from the light divider 4 is then increased.
The other light pulse obtained from the output terminal 4A circulates through the optical loop consisting of the light mixer 3, the optical isolator 7, the erbium-doped optical fiber 1, the optical switch 5, and light divider 4.
In this manner, the light pulse repeatedly circulates through the optical loop and a positive-feedback amplification is carried out. The level of amplitude of the light pulse is increased step by step every time the light pulse circulates through the optical loop.
On the other hand, when the light pulse is emitted from the output terminal 1B of the erbium-doped optical fiber 1, the energy accumulated in the fiber is decreased due to the light emission. Therefore, the level of amplitude of the light pulse P.sub.op decreases step by step due to the decrease of the energy in the erbium-doped optical fiber 1.
As a result, a light pulse P.sub.op which has a plurality of steps in the leading and trailing portions is obtained from the output terminal 4B. FIG. 6B shows a light pulse P.sub.op which is an example of a light pulse P.sub.op obtained from the output terminal 4B.
FIG. 7 shows a detailed waveform of the light pulse P.sub.opa. In FIG. 7, a time T.sub.a is determined by a time which is required for circulating a light pulse through the optical loop at one cycle. A time T.sub.b corresponds to a period during which the control signal S.sub.c remains at a high level as shown in FIG. 6A, i.e., a period during which the optical switch 5 remains in an ON-state and the optical loop is formed.
Moreover, there are cases in which a wide output light pulse P.sub.op is necessary. In order to make the pulse width of the light pulse P.sub.op longer, it is necessary to make the length of the optical loop (i.e., the propagation delay time of the signal light of the optical loop) longer.
However, if the length of the optical loop is increased, the steps in the waveform of the light pulse P.sub.op are expanded and the waveform is distorted as shown in FIG. 7.
If such a distorted light pulse is used for OTDR and the distorted light pulse is supplied to an optical system to be analyzed, a reflected light having a distorted waveform is observed. Therefore, it is difficult to accurately analyze the optical system.
In order to obtain a light pulse P.sub.op having no distortion, it is necessary to change the characteristics of the erbium-doped optical fiber 1 or to change the characteristics of the pumped-light source 2. However, a design which allows these kinds of changes is difficult to construct.